How would one test the quality of street cocaine?

How can I test the quality of cocaine, bought on the streets? I have heard of the numbing test, but that is not 100% fool proof test. I heard some people tell me it is in the color. Does anybody know the answer.

PS. I am aware that you get quality test kits.

Thanks
Update: How can I test the quality of cocaine, bought on the streets? I have heard of the numbing test, but that is not 100% fool proof test. I heard some people tell me it is in the color. Does anybody know the answer. I am aware that you get quality test kits. Thanks PS. Could we please have liberal people... show more How can I test the quality of cocaine, bought on the streets? I have heard of the numbing test, but that is not 100% fool proof test. I heard some people tell me it is in the color. Does anybody know the answer.

I am aware that you get quality test kits.

Thanks

PS. Could we please have liberal people answering this question with serious answers.
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